1. Field of the Invention
The present invention relates to an amplifier, an information communication apparatus and an amplification method of carrying out polar modulation, and more particularly, to an amplifier, an information communication apparatus and an amplification method of carrying out polar modulation while operating at high frequencies.
2. Related Art of the Invention
Battery life is a big problem for portable terminals, such as portable telephones. In particular, the amplifier in the transmission section of a wireless circuit consumes large power, affecting battery life significantly. Polar modulation is available as method of reducing the power consumption of the amplifier.
FIG. 7 shows the configuration of a conventional amplifier of carrying out polar modulation (refer to the Description of U.S. Pat. No. 6,366,177, for example). The conventional amplifier of carrying out polar modulation will be described below referring to FIG. 7.
The conventional amplifier comprises a modulation coder 701, a phase signal block 702, an amplitude driver 703, an amplification device 704, an amplitude monitor 711, a phase monitor 712 and a modification signal controller 713. The amplitude monitor 711, the phase monitor 712 and the modification signal controller 713 form a feedback loop 710.
The modulation coder 701 is a circuit of generating a phase signal and an amplitude signal. The phase signal block 702 is a circuit capable of phase modulation. The amplitude driver 703 is a circuit of receiving the amplitude signal and a level control signal, and generating an amplitude modulation signal from the amplitude signal according to the level control signal input. The amplification device 704 is a circuit of mixing the phase modulation signal with the amplitude modulation signal and simultaneously generating and amplifying a modulation signal.
In addition, the amplitude monitor 711 is a circuit of monitoring the amplitude of the modulation signal output from the amplification device 704 and outputting an amplitude signal. The phase monitor 712 is a circuit of monitoring the phase of the modulation signal output from the amplification device 704 and outputting a phase signal. The modification signal controller 713 is a circuit of controlling the amplitude driver 703 and the phase signal block 702 according to correction tables.
Next, the operation of the conventional amplifier having this configuration will be described.
The modulation coder 701 codes data or signals, such as sound, characters and images, input from the outside, and generates a phase signal and an amplitude signal.
The phase signal generated by the modulation coder 701 is input to the phase signal block 702 capable of phase modulation. Then, the phase signal block 702 carries out phase modulation using the phase signal input and generates a phase modulation signal. The phase modulation signal output from the phase signal block 702 is input to the amplification device 704 and amplified by the amplification device 704.
On the other hand, the amplitude signal generated by the modulation coder 701 is input to the amplitude driver 703. Furthermore, the amplitude driver 703 receives a level control signal simultaneously. The amplitude driver 703 generates an amplitude modulation signal according to the amplitude signal and the level control signal, and inputs the signal to the power supply of the amplification device 704.
In the amplification device 704, the phase modulation signal is mixed with the amplitude modulation signal, and a modulation signal is generated and amplified simultaneously. If the power consumption of the amplitude driver 703 can be reduced, the amplifier can obtain high operation efficiency.
Generally speaking, the amplification device 704 responds linearly to the phase modulation signal and the amplitude modulation signal; as a result, a distortion is generated in the modulation signal generated and amplified by the amplification device 704. Hence, a method of compensating for the distortion using the feedback loop 710 is adopted.
In the feedback loop 710, the modulation signal output from the amplification device 704 is monitored by the phase monitor 712 and the amplitude monitor 711 and branched into a phase signal and an amplitude signal. The phase signal and the amplitude signal branched are input to the modification signal controller 713. The modification signal controller 713 controls the amplitude driver 703 and the phase signal block 702 according to the correction tables, and adjusts the amplitude modulation signal and the phase modulation signal output from the amplitude driver 703 and the phase signal block 702, respectively, so that the modulation signal output from the amplification device 704 is not distorted.
In the case that a semiconductor transistor is used as the amplification device, it has large nonlinearity. The conventional method of correcting the distortion of the modulation signal, shown in FIG. 7, leads to the increase in circuit size and cost, and makes distortion compensation at speed sufficient for modulation speed difficult. Furthermore, if the compensation amount of the modulation signal at the modification signal controller 713 is large, the size of the correction tables becomes large, causing a problem of increasing memory size.
In other words, for the purpose of reducing the distortion of the modulation signal output from the amplification device, there is a problem that the increase in circuit size and cost cannot be avoided.
Furthermore, in the case that a heterojunction bipolar transistor or the like is used as the amplification device, the minimum collector voltage of generating the output power, that is, the threshold value of the collector voltage, changes with temperature; as a result, the distortion of the modulation signal output from the amplification device increases.
More specifically, in the case that a semiconductor transistor, such as a heterojunction bipolar transistor, is used as the amplification device, there is a problem that the distortion of the modulation signal is generated owing to temperature change.
For the purpose of solving the conventional problems described above, an object of the present invention is to provide an amplifier, an information communication apparatus and an amplification method of reducing the distortion of the modulation signal output from an amplification device while suppressing the increase in circuit size and cost.
Furthermore, another object of the present invention is to provide an amplifier, an information communication apparatus and an amplification method capable of reducing the distortion of the modulation signal owing to the change in the temperature of the amplification device.